Jest Matchers

For a given test like:

it("returns apple", () => {
    const fruit = getFruit();

    expect(fruit).toBe("apple");
})

Here the .toBe("apple") is the matcher.

Some common matchers:

• toBeUndefined, which matches only undefined
• toBeDefined, which matches anything else
• toBeTruthy, toBeFalsy
• toMatch, to check strings against regular expressions
• toContain, checking for items in arrays

See using matchers from the Jest documentation.

Floating point errors

JS infamously uses floating point arithmatic to handle decimal numbers. Meaning you can often be off by fractions of fractions.

To avoid this, use toBeCloseTo. Signature of which looks like:

function toBeCloseTo(expected: number, actual: number, precision?: number = 2)

Where precision is the number of decimal places to use for comparision. It defaults to 2 places.

See the Jest source for toBeCloseTo

Exceptions

To check for exceptions, you'll need to both wrap the excepting function in a anonymous function, and use toThrow.

const iThrowErrors = () => {
    throw new Error("Oh NO!");
}

it("throws errors", () => {
    // This passes
    expect(() => iThrowErrors()).toThrow():

    // This also passes!
    expect(() => iThrowErrors()).toThrow("Oh NO!"):
})

jest-dom

Common DOM matchers like .toBeVisible() or .toBeInTheDocument() come from the companion library jest-dom.

To install, install jest-dom to a dev dependency:

npm install -D jest-dom

And then add an import to your Jest setup script:

import "@testing-library/jest-dom";

// Add to jest.config.[j|t]s if not there already:
setupFilesAfterEnv: ['<rootDir>/path/to/jest-setup.[j|t]s']

See jest-dom on the Jest documentation